A motor vehicle drive train of the type indicated above is known from DE 199 34 696 A1 and DE 101 52 471 A1. In these known drive trains, the summation gear system is, in each case, made as a simple planetary transmission with a sun gear, a planetary gear carrier with a plurality of planetary gears and a ring gear. The ring gear constitutes the first input element and is connected in a rotationally fixed manner to the crankshaft of the combustion engine. The sun gear is the second input element and is connected in a rotationally fixed manner to the rotor of the electric machine. The planetary carrier is the output element and is connected in a rotationally fixed manner to the input shaft of the shift transmission. The bridging clutch is, in each case, arranged between the sun gear and the planetary carrier of the planetary transmission.
According to DE 199 34 696 A1, in contrast to the structure described above, in the drive train the bridging clutch is formed as a claw-type clutch so that the bridging clutch can only be engaged when the combustion engine and the input shaft of the shift transmission are turning at the same speed with the consequence that its use is only limited. To enable the motor vehicle to be driven by the electric machine alone, a directional idler is arranged between the crankshaft and part of the housing, which prevents the crankshaft from backward rotation so that the drive torque of the electric machine is supported against the housing. To enable the combustion engine to be started by the electric machine when the motor vehicle is at rest, another directional idler is arranged between the input shaft of the shift transmission and part of the housing, which prevents the input shaft from backward rotation so that the drive torque of the electric machine is again supported against the housing.
According to DE 101 52 471 A1, as also proposed by the present invention, in the drive train, the bridging clutch is made as a friction clutch so that the bridging clutch can be used in slipping operation to transmit a torque even when there is a speed difference between the input shaft of the shift transmission and the combustion engine. To enable an impulse start of the combustion engine by the electric machine when the motor vehicle is at rest and the transmission is in neutral, a further friction clutch is arranged between the input shaft of the shift transmission and part of the housing whereby, once the electric machine has reached a starting speed, the input shaft can be braked in order to start the combustion engine.
Below, in the description of the invention the point of departure taken as an example without restricting the scope of protection will be a drive train of largely identical structure, such that the bridging clutch is assumed to be a friction clutch made in particular as a wet-operating disk clutch, but which can also be made as a dry clutch. Alternatively to the known arrangement, the bridging clutch can also be positioned between the ring gear and the sun gear, i.e., between the crankshaft of the combustion engine and the rotor of the electric machine.
During normal driving operation the bridging clutch is fully engaged so that the planetary transmission is locked and rotates solidly. In this operating condition, the speeds and rotation directions of the combustion engine, the electric machine and the input shaft of the shift transmission are all identical. In this condition, the electric machine operates mainly as a generator for supplying the on-board electrical system but, in certain operating situations particularly during acceleration phases of the motor vehicle, it can also operate as a motor for a time.
With a drive train of such type, in certain operating situations such as during starting and after a shift operation, when the bridging clutch is disengaged, a speed difference occurs in the summation gear system which then has to be equalized, i.e., synchronized, in order to enable the bridging clutch to be engaged again with as little slip and wear as possible. The synchronization of a speed difference Dn=n_GE−n_VM so caused between the input shaft of the shift transmission and the combustion engine and, therefore, also a speed difference Dn_K acting at the bridging clutch, usually takes place by way of the electric machine with the bridging clutch fully disengaged in that, if the speed difference is negative (Dn<0), the electric machine produces a positive torque M_EM>0 that acts in the rotational direction of the crankshaft of the combustion engine and the input shaft of the transmission while, if the speed difference is positive (Dn>0), it produces a negative torque M_EM<0 that acts in opposition to the rotational direction of the crankshaft of the combustion engine and the input shift of the transmission.
If the electric machine cannot bring to bear the torque necessary for this, for example by virtue of its design or because the battery of the on-board electrical system is discharged, the possibility exists of assisting synchronization by a controlled engagement of the bridging clutch. When this is done, however, there is a disadvantageous delay followed by a jerk, since the bridging clutch first has to be brought from the fully disengaged condition to the contact point before a torque can be transmitted and synchronization can proceed.
On the other hand, the bridging clutch should be kept fully disengaged for as long as possible in order to avoid a more prolonged slipping phase combined with an undesired drag torque and thermal problems.
Against this background, the purpose of the present invention is to indicate a method whereby, with a drive train of the type described earlier, the synchronization process can be improved. Such a method should also be able to be used in drive trains that comprise comparable components but in a different drive coupling combination.